It is known in the art of medical radiography to employ intensifying screens to reduce the X-ray dosage to the patient. Intensifying screens absorb the X-ray radiations and emit electromagnetic radiations which can be better absorbed by silver halide emulsion layers. Another approach to reduce the X-ray dosage to the patient is to coat two silver halide emulsion layers on the opposite sides of a support to form a duplitized radiographic element.
Accordingly, it is a common practice in medical radiography to use a radiographic assembly consisting of a duplitized radiographic element interposed between a pair of front and back screens.
The typical structure of an intensifying screen comprises a support and a phosphor layer coated thereon. The phosphor layer comprises a fluorescent substance able to emit light when exposed to X-ray, mixed with a binder.
The screens most widely used in radiography can be classified according their characteristic speed in (1) fast screens, (2) average screens, and (3) slow screens. The speed of a screen is directly proportional to the thickness of the phosphor layer and to the grain size of the fluorescent substance.
A well known problem of intensifying screens relates to the sharpness of the resulting image. The presence of the intensifying screen reduces the sharpness of the resulting image, in particular When fast screens are used. This is due to the large fluorescent crystals and to the high thickness employed to realize the fast screens. On the other hand, there are situation wherein a reduction of exposure is of prime importance in spite of some sacrifice in image sharpness.
To increase the amount of fluorescent radiation which impinges the radiographic film interposed between screens, and then to increase the speed of the screen without a significant loss in sharpness, a reflective layer is usually interposed between the fluorescent layer and the support. Titanium dioxide is usually employed as a reflective substance.
However, a part of the far UV light emitted by the fluorescent substance usually employed for UV-blue emitting screens is poorly reflected by titanium dioxide. This means a speed loss which can be calculated in the order of 0.2-0.3 LogE.
In order to recover such a loss of speed, different reflective layers have been suggested, but only a marginal increase of speed has been obtained, probably due to the absorption of the far UV light by the fluorescent layer itself.
DE 2,807,398 discloses an X-ray fluorescent screen comprising colored plastic particles which contain two dyestuffs, one emitting in the red portion of the spectrum and the other emitting a shorter wavelength. Said dyestuffs are particularly effective in combination with La.sub.2 O.sub.3 -LaBr:Tb phosphors.
U.S. Pat. No. 4,028,550 discloses an X-ray screen comprising europium activated fluorohalide phosphors. The use of an aminocoumarine brightening agent is suggested for reduced mottle and improved image quality.
U.S. Pat. No. 4,675,271 discloses an X-ray permanent screen comprising europium and strontium activated fluorohalide storage phosphors for use in digital radiography. The use of diaminostilbene disulfonic acids is suggested to reduce the dispersion of the scanning beam in the persistent layer, providing better picture sharpness.